Evaporator tube assembly



Feb. 27, 1968 E. KUMM 3,

EVAPORATOR TUBE AS 5 EMBLY Filed April 16, 1964 INVENTOR.

EMERSON L. KUMM A TTOR/VE Y United States Patent @fifice 3,37%,635 Patented Feb. 27, 1968 3,370,635 EVAPORATOR TUBE ASSEMBLY Emerson L. Kumm, Scottsdale, Ariz., assignor to The Garrett Corporation, Los Angeles, Calif., a corporafion of California Fiied Apr. 16, 1964, Ser. No. 369,267 2 Claims. (Cl. 15913) ABSTRACT 6F THE DISCLGSURE An evaporator tube assembly including a main tube adapted to extend vertically from an inlet tank through a heating tank to an outlet tank, the upper end of the main tube containing a liquid distributing inner tube having a plurality of ridges on the outer surface for cooperation with the main tube to form a plurality of spiral grooves extending around the inside of the tube wall. These grooves are open to the inlet tank at the top and are disposed below the liquid level which in turn is below the upper end of the inner tube. The portion of the inner wall of the main tube immediately below the lower end of the inner tube is smooth, the balance being formed with shallow, closely spaced, axially extending grooves. The smooth portion of the inner wall is located to receive a minimum of heat to reduce evaporation at this point and may be insulated if desired. In operation, liquid flows through the spiral grooves, establishes a substantially uniform film over the smooth wall area and flows vertically down the remaining portion of the tube to be vaporized by heat applied to the exterior of the tube in the heating tank.

The present invention relates generally to evaporators, and more particularly to means for establishing and maintaining over the interior surface of an evaporator tube or the like a thin, continuous water film from which the liquid is evaporated.

In a broad aspect, the present invention is concerned with the establishment and maintenance of a water film over the interior surface of a tubular member, and since it has been developed primarily for use in a tube-type evaporator, it will be shown and described in that embodiment, but without intending that the invention be necessarily limited thereto.

In certain types of evaporators, a water film is maintained over the interior surface of a tube and heat is transmitted to the film through the tube wall from an exterior source in order to supply heat to the water for evaporation of a portion thereof. In a typical evaporator of known design, it has been found that the quantity of Water evaporated per pass through a tube amounts to Only about 5% of the water introduced into the tube. It has also been found that the percentage of water evaporated per pass through can be increased by reducing the film thickness.

Reduction in the film thickness has the effect of reducing" the mass of water passing through the tube in a unit time. Decrease in the film thickness also permits a greater heat flow from the exterior surface of the tube since a major portion of the total resistance to the heat flow varies directly with the film thickness under typical laminar flow thin film conditions. The larger heat flow applied to a smaller mass of water flow results in a greater percentage of the water being evaporated with a thinner film. Experiments have indicated that the evaporation rate may be increased as much as two to five times over that available with known types of equipment, which may be distinguished from the present invention by referring to them as employing thick water films.

In order to maintain high eficiency and long life of the evaporator tubes, the Water film must be continuous and at all times cover the entire internal surface of the tube. While the need for continuity of the film places some limitation upon the minimum thickness of the film, it has been found that one of the major sources of discontinuity in the film is not thinness of the film itself but rather the defective initial formation of the film by not evenly distributing the water over the internal surface of the tube. This often results in thin areas that produce dry spots which, once established, continue to maintain themselves with the result that there is scale or corrosion occurring at the wet-dry junction. Sometimes, at thin areas of the water film, salts may deposit from the water solution which produce a roughness on the interior of the tube, preventing reestablishment of the continuous water film. Uniform distribution of the water initially over the tube surface results in the formation of a continuous film which, when once established, has been found to be highly effective in maintaining itself as it is able to preent the formation of dry spots and is able to flush away small particles of foreign matter.

Thus, it is a general object of the present invention to provide means for forming a thin, substantially uniform film of water or other liquid over the interior surface of a tubular member.

It is also an object of the present invention to establish and maintain a thin but continuous water film over the interior surface of an evaporator tube in order to increase the evaporating efficiency and rate of an evaporator.

A further object of the present invention is to provide means for forming over the interior of a tubular member a thin, continuous water film which resists scaling or corrosion of the tubular member and minimizes the adhesion of solid particles which, if retained, interrupt the continuity of the film.

These and other objects of the present invention are attained by providing an evaporator tube, which is open at both ends and is disposed with its axis substantially vertical, with means for introducing a liquid into the upper end of the tube in a plurality of spiral streams that are directed downwardly over the interior surface of the evaporator tube and which blend together to form a continuous film of liquid, of substantially uniform thickness, over the interior surface of the tube. The film flows by gravity to the bottom of the tube.

More specifically, such means comprises a tubular insert in the upper end of the evaporating tube, forming, in cooperation with the evaporating tube, a plurality of spiral passages which introduce water into the upper end of the evaporating tube in a plurality of spiral streams which distribute the incoming water evenly over the tube surface. While the evaporating tube is preferably metal in order to have a high rate of heat transfer, the insert is preferably molded of a synthetic resin having a relatively low rate of thermal conductivity. In addition to providing a desirable reduction in the rate of heat transfer to the incoming Water streams, the synthetic resin also provides a smooth surface which minimizes the tendency of any foreign particles or deposited salts to adhere to the insert.

How the above objects and advantages of the present invention, as well as others not specifically mentioned herein, are attained will be better understood by reference to the following description and to the annexed drawing, in which:

FIG. 1 is a schematic vertical section through an evaporator of the vapor-compression cycle type;

PEG. 2 is an enlarged fragmentary section through the upper end of an evaporator tube assembly, as on line 22 of FIG. 1; and

FIG. 3 is a fragmentary side elevation of a portion of The static hydraulic head upon the water entering;

the 'tube assembly showing a variational form of the present invention.

The evaporator indicated generally at it} in FIG. 1 is designed particularly to distill water using a vapor-com- ;pression cycle, a cycle which is itself well known in the -''art. The apparatus comprises an upper tank 1-1 having an inlet 12 through which water to be distilled enters the upper tank. At some point above the water level therein, and here shown as being on the top wall of the evaporator, the upper tank is provided with outlet 14 for water vapor.

Within the evaporator and below the upper tank is an intermediate tank 16 having an inlet 17 through which heated and compressed water vapor enters the intermediate tank. After being cooled and condensed, this water vapor is withdrawn in liquid form from the intermediate tank through its outlet 18. At the bottom of the apparatus is bottom tank 20 which serves as a reservoir or receiver for the water leaving the evaporating tubes, to be described. The residual water entering bottom tank 20 is discharged from the apparatus through outlet 22.

Evaporation of water or other liquid is carried on inside evaporating tube 24. The present apparatus is shown as if it comprised only a single evaporating tube, but it will be understood that in a commercial design the apparatus comprises a large number of duplicate tubes of which the single tube shown is typical. Tube 24 is open at both ends, being in communication at its upper end with the body of water contained in upper tank 11 and its lower end with lower tank 20. Tube 24 thus passes through the partition walls 25 and 26 which respectively separate the upper tank and the bottom tank from the intermediate tank.

Mounted within the upper end of evaporating tube 24 and concentric therewith is an inner tube 30 which is open at both ends and is of sufficient length to project above the water in tank 11, in order to discharge from the upper end vapor formed within tube 24. Tube 39 is also means for introducing the water into the upper end of the evaporating tube in a plurality of spiral streams which are directed over the interior surface of the evaporating tube, as will now be explained.

The upper portion of tube 30 has an exterior diameter smaller than the interior diameter of tube 24 so that there is an annular space, as shown at 31, between the inner tube and the evaporating tube. However, the lower portion of the inner tube is provided with a plurality of spiral ridges 32 which form between successive ridges a plurality of spiral grooves 33 in the exterior surface of the inner tube. The maximum diameter of ridges 32 is such that the ridges engage the interior surface of tube 24 so that tube 24 closes the open side of grooves 33 to provide a pluralityof spiral liquid passages, all of which terminate at the bottom of the tube 30, substantially at a common level with respect to tube 24.

The inner vapor discharge tube may be supported inside the evaporating tube in any suitable way. However, it is convenient to provide a plurality of outwardly projecting ribs 35 which engage and rest upon the upper end of the evaporating tube to hold the inner tube in place. Ribs 35 may be appropriately positioned on tube 30 in order to hold the upper end of the tube at the proper level.

In operation, water enters the upper end of the evap- :orating tube inthe annular space 31 between the evaporating tube .and tube 30 and then flows downwardly through the plurality of spiral passages 33 which are formed by the grooves in the inner tube and in cooperation with the inner surface of the evaporating tube. The spiral streams then are discharged to fiow over the inner surface of the evaporating tube, the plurality of streams being discharged at difierent positions around the tube. The tangential component of their motion causes all the streams to blend together before they flow downwardly under the influence of gravity.

grooves 33 depends upon the height of the water level in tank 11 above the inlet ends of the grooves. This can be closely controlled in order to provide a known pressure on the water at entry. The friction loss encountered by a water stream passing through a groove can be regulated by controlling the cross-sectional area and the length of the groove so that the velocity of the streams' at discharge can be closely regulated. Several grooves are preferred, it being contemplated that from three to six grooves will provide optimum results, allowing the several streams at discharge from the lower end of the tube to flow around the inner surface of evaporating tube a sufficient horizontal distance that all of the streams blend into a thin film of substantially uniform thickness before the water film moves directly down the inside of the tube.

In order to reduce the friction loss on the streams of water in grooves 33, it is preferred that the inner surface of tube 24 be smooth over that area which is opposite the grooves 33 and also fora short distance below the lower end of tube 30, as indicated in the zone 36. This zone 36 may be typically of the order of one-fourth inch 7 to one-half inch long, although a longer zone is also satisfactory. While the entire inner surface of tube 24 may be smooth, it is preferred that below thezone 36 the inner surface of the evaporating tube may be provided with a series of small, axially extending grooves 40, as shown in FIGS. 1 and 2. These grooves are disclosed in greater detail and claimed in the copending application Ser. No. 230,858 filed jointly by me with Yasutoshi Senoo on Oct. 16, 1962, for Saline Water Conversion System.

These grooves 40 extend from the end of smooth zone 36 to the bottom of tube 24 and have been found effective in maintaining the water fihn evenly distributed and unbroken over the lower portion of the evaporating tube.

Heat is supplied to the exterior surface of evaporator tube 24 by compressed, heated water vapor introduced into intermediate tank 16 through inlet 17. The hot vapor heats the wall of the tube and the heat is transmitted through the tube wall to the water film on the interior surface thereof. For this reason, tube 24 is preferably made of metal as this material has a relatively high CO- eflicient of thermal conductivity.

However, it is not desirable for evolution of vapor from the water film to take place before the water film is well formed, and for this reason the upper. portion of the tube, above zone 36, is. preferably not subjected to exterior heat. Accordingly, partition 25 is preferably located near the lower end of spiral grooves 33. In this way the body of water in upper tank 11 acts as a water jacket surrounding the spiral passages to prevent formation of vapor from the Water until after the water has been discharged from grooves 33.

It may be desired to further retard the formation of vapor in the upper end of evaporating tube 24, and for this purpose the outer surface of tube 24 may be coated for a short distance with a layer of synthetic resin or plastic which exhibits a low coefficient of thermal conductivity in order to retard the flow of heat through the.

tubewall to the water film. Such a film, at 42 in FIG. 3, may extend down below zone 36 as far as desired.

Also, in order to retard the transfer of heat tothe. incoming water stream, the entire vapor discharge tube and water inlet means is preferably made of a material having a low coeflicient of thermal conductivity. Such a material may be a molded synthetic resin or plastic. Another advantage of this type of material for the tube inserted in the evaporating tube is the fact that a smooth surface can be produced which has very little tendency to cling to particles of foreign matter such as solids that may be contained in the water or salts which may be deposited out as a result of evaporation. For this reason one of the various nylons or tetrafluoroethylene compounds may be advantageously used for the tube 30.

It may be desired in some installations to adjust the effective length of passages 33 in order to change the resistance encountered by the streams of water as they flow through the passages. Although other means for adjustably mounting a tube 30 on the upper end of an evaporating tube may be devised, a simple and illustrative arrangement is shown in FIG. 3.

In this figure, tube 30 is supported by frictional engagement with a surrounding collar 45 which slides on the smooth upper portion of tube 30 with sufiicient friction to support the tube at any adjusted position. Collar 45 is provided with a plurality of spaced lugs 46 which rest upon the upper end of the evaporating tube to support tube 30. In this way, the length of tube 30 and passages 33 submerged below the level of water in tank 11 may be adjusted as desired.

It will be apparent that various changes in the detailed construction and arrangement of the elements comprising the present invention may occur to persons skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the foregoing description is considered illustrative of rather than limitative upon the present invention.

I claim:

1. A tube assembly for an evaporator, comprising:

(a) an evaporating tube open at both ends and disposed with its axis substantially vertical;

(b) an inner tube open at both ends and concentric with the evaporating tube, said inner tube having a plurality of spaced, narrow ridges extending spirally around the outer side and cooperating with the wall of said evaporating tube to form passages for slowly admitting a liquid into the upper end of said evaporating tube and distributing the same over the interior surface of the evaporating tube in a thin film, the ridges on said inner tube being relatively low in height to limit the depth of the spiral grooves and restirct the volumetric flow of liquid to the interior surface of the evaporating tube, the interior surface of the evaporating tube being smooth opposite the spiral grooves and a predetermined distance below said inner tube, the interior surface of the evaporating tube having axially extending grooves below said predetermined smooth portion; and

(c) a layer of synthetic resin coating on the outside of the tube at the smooth interior portion only to inhibit the application of heat thereto.

2. A tube assembly for an evaporator, comprising:

(a) a metal evaporating tube open at both ends and disposed with its axis substantially vertical; and

(b) an inner tube disposed in the upper end of said evaporating tube, said inner tube being of molded synthetic resin having high resistance to corrosion and adhesion of foreign matter, said inner tube having a plurality of spaced, narrow ridges extending spirally around the outer side and cooperating with the wall of said evaporating tube to form passages for slowly admitting a liquid into the upper end of said evaporating tube and distributing the same over the interior surface of the evaporating tube in a thin film.

References Cited UNITED STATES PATENTS 538,557 4/1895 Theisen 202236 X 1,164,413 12/1915 Shaw 159-13 1,557,838 10/1925 Hiller 15913 3,244,601 4/ 1966 Diedrich 20310 3,247,888 4/ 1966 Mueller et a1. 15913 FOREIGN PATENTS 77,139 7/ 1919 Austria. 308,727 3/ 1929 Great Britain.

NORMAN YUDKOFF, Primary Examiner.

DAVID EDWARDS, Assistant Examiner. 

